Ordinary spark plugs have an external thread on a metal outer shell with a hexigonal head. They seat in a threaded bore of the cylinder head with a deformable gasket seal. Complete sealing and correct positioning of the spark in the combustion chamber requires a precise torque. Excessive torque or incorrect positioning may strip the threads in the cylinder head, requiring expense repairs. Space for tools such as torque wrench is limited in many engine compartments and access is often awkward. All of the problems associated with spark plug replacement are magnified in auto racing competition with engine heat and time constraints added.
U.S. Pat. No. 5,186,132 issued Feb. 16, 1993 to Runge teaches a plug-in spark plug that requires a special bore in the cylinder head with a retaining groove for engaging a locking circlip. It requires some sort of tool fitting in a groove to forcefully pull the plug out and a tool for engaging the circlip to reduce its diameter to disengage it from the retaining groove. It would be desirable to have a system that would operate with conventionally bored and threaded cylinder heads, since most consumers don't replace their own plugs and we could not expect the engine manufacturers to provide the special cylinder heads.
U.S. Pat. No. 3,747,583 issued Jul. 24, 1973 to Gerard Georges and Erich Spengler teaches a quick insertion spark plug arrangement in which an outer sleeve screws into the threaded bore in a cylinder head. The sleeve has an inner profile that cooperates with an outer profile of the plug.
In a first rotary position of the plug, it may be moved axially into and out of the sleeve. When the inserted plug is rotated about its axis a finite angle by a special tool, the inner and outer profiles cooperate to lock the position of the plug against axial movement in a threaded engagement.
Quick disconnect couplings for joining conduits for high pressure fluids are exemplified by U.S. Pat. Nos. 3,162,470 issued Dec. 22, 1964 to Davidson and SWAGELOK (Registered Trademark) full flow quick-connect coupling QF series made by the Swagelok Company of Hudson, Ohio. These use a hand-operated sliding locking sleeve that requires no tool for engagement and disengagement. This style of connection has not been applied to spark plugs.